Acetylene (or ethyne, HC≡CH) has long been recognized as one of the few compounds that can be made directly at high selectivity from methane but the conditions of that manufacture have placed it beyond commercial practicality for other than high cost, specialty production. Acetylene can be converted to a number of other desirable hydrocarbon products, such as olefins and vinyls. One of the biggest impediments to producing acetylene from methane feeds has been the very high temperatures required to produce high-yield conversion of methane to acetylene. Many of the desired products that could be manufactured from the produced acetylene are today instead being produced via more economical processes, such as thermal cracking of higher molecular weight hydrocarbon feeds such as ethane and naphthas, in thermal crackers. The higher molecular weight feed crack at lower temperatures than methane. Equipment, materials, and processes were not previously identified that could continuously withstand the high (>1600° C.) temperatures required for methane pyrolysis. Pyrolyzing large quantities of methane had been considered much too costly and impractical due to the special types and costs of equipment that would be required. The developed processes for producing acetylene have all operated commercially at lower temperatures for steam cracking of higher weight hydrocarbon feeds.
It is known that acetylene may be manufactured from methane in small amounts or batches, using a high temperature, short contact time, yielding a mixture of acetylene, CO, and H2. Comprehensive discussions are provided in the Stanford Research Institute report entitled “Acetylene,” a Process Economics Program, Report No. 16, September 1966, and in the Fuel Processing Technology publication (42), entitled “Pyrolysis of Natural Gas: Chemistry and Process Concepts,” by Holmen, et. al., 1995, pgs. 249-267. However, the known processes are inefficient, do not scale well, and are generally only useful for specialty applications.
The known art discloses that to efficiently obtain relatively high yields of acetylene, such as in excess of 50 wt % or more preferably in excess of 75 wt % acetylene from the methane feed, temperatures are required to be in excess of 1500° C. and preferably in excess of 1600° C., and with short contact times (generally<0.1 seconds) to prevent breaking the acetylene into elemental carbon and hydrogen components. Such temperature and processes have largely been unattractive due to the degradation of the equipment utilized. Virtually any metal components that are exposed to such temperatures will be costly and will unacceptably degrade.
In addition to the above references, U.S. Pat. No. 2,813,919 discloses acetylene manufacture from methane in a reverse-flow reactor (a regenerative furnace), operating at temperatures of typically 2500° F. (1370° C.), but up to 3000° F. (1650° C.). U.S. Pat. No. 2,885,455 discloses a reverse-flow reactor (a regenerative pebble-bed reactor) for production of acetylene from light hydrocarbons. Ethane and propane feeds are discussed and claimed; methane is not mentioned. Reaction temperatures up to 3000° F. (1650° C.) and contact times of 0.1 second or less are disclosed. U.S. Pat. No. 2,886,615 describes a reverse-flow reactor (a regenerative pebble-bed reactor) useful for processing hydrocarbon feed stocks (including natural gas) with hydrogen reactant to prepare olefins, acetylenes, and other product. Temperatures in excess of 3000° F. (1650° C.) and reaction times of 0.001 to 1 second are disclosed. The improvement taught is a secondary heat reservoir.
U.S. Pat. No. 2,920,123 describes pyrolysis of methane to produce acetylene at temperatures of 2820° F. (1550° C.) to 2910° F. (1600° C.) and contact time in the range of 0.004 to 0.015 seconds. The exemplified reactor is an electrically heated ceramic tube, and the soft carbon produced as a byproduct under these conditions is removed by oxidation after 5 seconds of feed.
U.S. Pat. No. 3,093,697 discloses a process for making acetylene by heating a mixture of hydrogen and a hydrocarbon stock (e.g., methane) at a reaction temperature that is dependent upon the particular hydrocarbon employed, for about 0.01 to 0.05 second. The reference indicates that a reaction temperature of 2700° F. to 2800° F. (about 1482° C. to about 1538° C.) is preferred for methane and that lower temperatures are preferred for higher molecular weight hydrocarbons.
U.S. Pat. No. 3,156,733 discloses a process for the pyrolysis of methane to acetylene and hydrogen. The process involves heating a methane-containing stream in a pyrolytic reaction zone at a maximum temperature above 2730° F. (1500° C.) and sequentially withdrawing a gaseous product from the reaction zone and quenching the product rapidly to a temperature of about 1100° F. (600° C.) or less. U.S. Pat. No. 4,176,045 discloses a process for the production of olefins by steam-cracking normally liquid hydrocarbons in a tubular reactor wherein the residence time in the tubes is from about 0.02 to about 0.2 second and the formation of coke deposits in the tubular reactor is minimized. U.S. Pat. No. 4,929,789 discloses a process for pyrolyzing or thermal cracking a gaseous or vaporized hydrocarbon feedstock using a novel gas-solids contacting device and an oxidation catalyst. U.S. Pat. No. 4,973,777 discloses a process for thermally converting methane into hydrocarbons with higher molecular weights using a circulating methane atmosphere in a ceramic reaction zone.
Chemical Economy and Engineering Review, July/August 1985, Vol. 17, No. 7.8 (No. 190), pp. 47-48, discloses that furnaces have been developed commercially for steam cracking a wide range of liquid hydrocarbon feedstocks using process reaction times in the range of 0.05 to 0.1 second. This publication indicates that the use of these furnaces permits substantial increases in the yield of olefins (i.e., ethylene, propylene, butadiene) while decreasing production of less-desirable co-products. GB 1064447 describes a process for production of acetylene from pyrolysis of methane and hydrogen (1:1 to 39:1 H2:CH4; clm. 9) in an electrically heated reactor, and quenching with a dry, oxygen-free gas stream. The maximum temperature is 1450 to 2000° C. (preferably 1450 to 1750° C.; clm. 2).
The “Wulff” process represents one of the more preferred commercial processes for generation of acetylene. The Wulff process includes a reverse-flow thermal pyrolysis process and began development in the 1920's. Various related processes operated commercially up to about the 1960s. These processes typically used feeds heavier than methane and thereby operated at temperatures of less than 1500° C. The most complete description of the Wulff process is provided in the Stanford Research Institute's “Acetylene”, Process Economics Program Report Number 16 (1966). Among the relevant patents listed in this report are U.S. Pat. Nos. 2,319,679; 2,678,339; 2,692,819; and 3,093,697, discussed above. It is believed that all commercial acetylene plants operated on feeds of ethane, naphtha, and/or butane, but that none have successfully operated on methane feeds. Wulff discloses a cyclic, regenerative furnace, preferably including stacks of Hasche tiles (see U.S. Pat. No. 2,319,679) as the heat exchange medium. However, to contain the location of the reaction heat generated by the exothermic combustion process, one of either the fuel or oxygen is introduced laterally or separately into the central core of the reactor where it mixes with the other reaction component. The other reaction component is preferably introduced through the reactor tiles to cool the reactor quench section. Thereby, combustion may occur in a known location within the reactor. However, this also exposes the lateral injection nozzles or ports to the combustion product, including the extremely high temperature needed to crack methane feeds. Degradation in nozzle performance, shape, and/or size consequently made it extremely difficult to control flame shape, temperature, and efficiency. Although some of the Wulff art disclose use of various refractory materials, a commercially useful process for methane cracking was not achieved utilizing such materials. Also, a further drawback of the Wulff process is that the laterally or separately introduced portion of exothermic reactant is not utilized for quenching the recuperation reactor bed.
Regenerative reactors, including reactors such as disclosed by Wulff, are typically used to execute cyclic, batch-generation, high temperature chemistry. Typically, regenerative reactor cycles are either symmetric (same chemistry or reaction in both directions) or asymmetric (chemistry or reaction changes with step in cycle). Symmetric cycles are typically used for relatively mild exothermic chemistry, examples being regenerative thermal oxidation (“RTO”) and autothermal reforming (“ATR”). Asymmetric cycles are typically used to execute endothermic chemistry, and the desired endothermic chemistry is paired with a different chemistry that is exothermic (typically combustion) to provide heat of reaction for the endothermic reaction. Examples of asymmetric cycles are Wulff cracking processes and pressure swing reforming processes.
As mentioned above, regenerative reactors are known that separately deliver a stream of fuel, oxidant, or a supplemental amount of one of these reactants, directly to a location somewhere in the heat generation region of the reactor. Although this may defer or control location of combustion, that process limits the cooling of the quench regions of the reactor, due to not having that stream pass through regenerative beds or regions. This can result in expanding heat zones loss of reaction control.
The reactor heat generation region is typically a region of the reactor system that is in between two regenerative reactor beds or regions, with the main regenerative flow passing from one of these bodies to the other. In most cases, this lateral stream is introduced via nozzles, distributors, or burners (e.g., Wulff) that penetrate the reactor system using a means that is generally perpendicular to flow direction and usually through the reactor vessel side wall. In large scale operations, such methods are impermissibly inefficient and costly. For example, during the exothermic step in a conventional Wulff cracking furnace, air flows axially through the regenerative bodies, and fuel is introduced via nozzles that penetrate the side of the furnace, to combine with air (combusting and releasing heat) in an open zone between regenerative bodies. In a conventional symmetric RTO application, a burner is placed to provide supplemental combustion heat in a location in between two regenerative bodies. The burner combusts fuel from outside the reactor, either with the air passing through the regenerative bodies, or using external air. Additional measure must be made to ensure adequate and timely quenching of the synthesized product, and to adequately cool the bed before the next cycle begins.
Attempts have been made to introduce a reactant of the exothermic step to a location in the middle of the regenerative reactor via conduits that are positioned axially within one or more of the regenerative bodies. For example, Sederquist (U.S. Pat. No. 4,240,805) uses pipes that are positioned axially within the regenerative bed to carry oxidant (air) to locations near the middle of the regenerative flow path.
All of these previously known systems suffer disadvantages that render the same inefficient and unpractical in any but very specialized, small scale operations with methane feeds. Positioning nozzles, distributors, or burners in the middle of the regenerative flow path of the reactor diminishes the durability and control of the reactor system. Nozzles, distributors, and/or burners all rely on carefully-dimensioned passages to regulate flow in a uniform manner, or to create the turbulence or mixing required to evenly distribute the heat that results from the exothermic reaction they support. These nozzles, distributors, and/or burners are located at the highest-temperature part of the reactor. It is very difficult to fabricate and maintain carefully-dimensioned shapes for use at high temperatures. If the nozzles or distributor loses its carefully-dimensioned shape, it will no longer produce uniform flame temperatures.
A further disadvantage of separately or laterally introducing one or more reactant directly into the middle or heat region of the regenerative flow path of the reactor is that such an arrangement bypasses that reactant around the regenerative flow path. In addition to not quenching the quench portion of the reactor, such approach also eliminates preheating that reactant stream. The fundamental purpose of a regenerative reactor system is ideally to execute reactions at high efficiency by recuperating product heat directly into feeds. Bypassing some fraction of the feed to the reactor around the regenerative system thus reduces the efficiency potential of the reactor system and can lead to expanded heat zones and feed conversion reactions that last too long.
All of the known art disclose processes, methods, and equipment that are unsuitable for continuous, high efficiency operation at the necessarily high temperature, due to complexity and thermal degradation of equipment. Also, the known processes do not reliably provide methods or means for continuously controlling the location and dissipation of the created heat, resulting in either hot spots, undesired thermal migration, and/or inefficient processes. What is needed is an efficient and cost-effective way to pyrolyze methane to acetylene at relatively high yield, selectivity, and efficiency, in a manner that is competitive with pyrolyzing other hydrocarbon feeds to acetylene.